The present invention is directed to the field of sails and methods for their manufacture.
Sails can be flat, two-dimensional sails or three-dimensional sails. Most typically, three-dimensional sails are made by broadseaming a number of panels. The panels, each being a finished sector of sailcloth, are cut along a curve and assembled to other panels to create the three-dimensional aspect for the sail. The panels typically have a quadrilateral or triangular shape with a maximum width being limited traditionally by the width of the roll of finished sailcloth from which they are being cut. Typically the widths of the sailcloth rolls range between about 91.5 and 137 centimeters (36 and 58 inches).
Sailmakers have many restraints and conditions placed on them. In addition to building products which will resist deterioration from weather and chafe abuses, a goal of modem sailmaking is to create a lightweight, flexible, three-dimensional air foil that will maintain its desired aerodynamic shape through a chosen wind range. A key factor in achieving this goal is stretch control of the airfoil. Stretch is to be avoided for two main reasons. First, it distorts the sail shape as the wind increases, making the sail deeper and moving the draft aft. This creates undesired drag as well as excessive heeling of the boat. Second, sail stretch wastes precious wind energy that should be transferred to the sailcraft through its rigging.
Over the years, sailmakers have attempted to control stretch and the resulting undesired distortion of the sail in three basic ways.
The first way sailmakers attempted to control sail stretch is by using low-stretch high modulus yarns in the making of the sailcloth. The specific tensile modulus in gr/denier is about 30 for cotton yarns (used in the 1940's), about 100 for Dacron.RTM. polyester yarns from DuPont(used in the 1950's to 1970's), about 900 for Kevlar.RTM. para-aramid yarns from DuPont (used in 1980's) and about 3000 for carbon yarns (used in 1990's).
The second basic way sailmakers have attempted to control sail stretch has involved better yam alignment based on better understanding of stress distribution in the finished sail. Lighter and yet lower-stretch sails have been made by optimizing sailcloth weight and strength and working on yarn alignment to match more accurately the encountered stress intensities and their directions. The efforts have included both fill-oriented films. With better understanding of the stress distribution, sailmaking has evolved towards more sophisticated panel-layout constructions. Up until the late 1970's, sails were principally made out of narrow panels of fill-oriented woven sailcloth arranged in cross-cut construction where the majority of the loads were crossing the seams and the width of the narrow panels. With the appearance of high-performance yarn material, like Kevlar, stretch of the numerous horizontal seams in the sails became a problem. To solve this and to better match the yarn alignment with the load patterns, an approach since the early 1980's has been to arrange and seam narrow panels of warp-oriented sailcloths in panel-layout constructions known as "Leech-cut" and later more successfully in the "Tri-radial" construction. The "Tri-radial" construction is typically broken into several sections made from narrow pre-assembled radiating panels. The highly loaded sections of the sail such as the clew, the head and the leech sections are typically made with radial panels cut from heavy sailcloth. The less loaded sail sections, such as the luff and the tack sections, are made with panels cut from lighter sailcloth. This approach, unfortunately, has its own drawbacks. Large sails made this way can have up to, for example, 120 narrow panels which must be cut and broadseamed to each other with great precision to form the several large sections. These large sections of pre-assembled panels are then joined together to form the sail. This is extremely time-consuming, and thus expensive, and any lack of precision often results in sail-shape irregularities. The mix of types of sailcloths used causes the different panels to shrink at different rates affecting the smoothness of the sail along the joining seams of the different sections, especially over time.
An approach to control sail-stretch has been to build a more traditional sail out of conventional woven fill-oriented sailcloth panels and to reinforce it externally by applying flat tapes on top of the panels following the anticipated load lines. See U.S. Pat. No. 4,593,639. While this approach is relatively inexpensive, it has its own drawbacks. The reinforcing tapes can shrink faster than the sailcloth between the tapes resulting in severe shape irregularities. The unsupported sailcloth between the tapes often bulges, affecting the design of the airfoil.
A further approach has been to manufacture narrow cross-cut panels of sailcloth having individual laid-up yarns following the load lines. The individual yarns are sandwiched between two films and are continuous within each panel. See U.S. Pat. No. 4,708,080 to Conrad. Because the individual radiating yarns are continuous within each panel, there is a fixed relationship between yarn trajectories and the yarn densities achieved. This makes it difficult to optimize yarn densities within each panel. Due to the limited width of the panels, the problem of having a large number of horizontal seams is inherent to this cross-cut approach. The narrow cross-cut panels of sailcloth made from individual spaced-apart radiating yarns are difficult to seam successfully; the stitching does not hold on the individual yarns. Even when the seams are secured together by adhesive to minimize the stitching, the proximity of horizontal seams to the highly loaded corners can be a source of seam, and thus sail, failure.
A still further approach has been to manufacture simultaneously the sailcloth and the sail in one sector on a convex mold using uninterrupted load-bearing yarns laminated between two films, the yarns following the anticipated load lines. See U.S. Pat. No. 5,097,784 to Baudet. While providing very light and low-stretch sails, this method has its own technical and economic drawbacks. The uninterrupted nature of every yarn makes it difficult to optimize yarn densities, especially at the sail corners. Also, the specialized nature of the equipment needed for each individual sail makes this a somewhat capital-intensive and thus expensive way to manufacture sails.
The third basic way sailmakers have controlled stretch and maintained proper sail shape has been to reduce the crimp or geometrical stretch of the yarn used in the sailcloths. Crimp is usually considered to be due to a serpentine path taken by a yarn in the sailcloth. In a weave, for instance, the fill and warp yarns are going up and down around each other. This prevents them from being straight and thus from initially fully resisting stretching. When the woven sailcloth is loaded, the yarns tend to straighten before they can begin resist stretching based on their tensile strength and resistance to elongation. Crimp therefore delays and reduces the stretch resistance of the yarns at the time of the loading of the sailcloth.
In an effort to eliminate the problems of this "weave-crimp", much work has been done to depart from using woven sailcloths. In most cases, woven sailcloths have been replaced by composite sailcloths, typically made up from individual laid-up (non-woven) load-bearing yarns sandwiched between two films of Mylar.RTM. polyester film from DuPont or some other suitable film. There are a number of patents in this area, such as Sparkman EP 0 224 729, Linville U.S. Pat. No. 4,679,519, Conrad U.S. Pat. No. 4,708,080, Linville U.S. Pat. No. 4,945,848, Baudet U.S. Pat. No. 5,097,784, Meldner U.S. Pat. No. 5,333,568, and Linville U.S. Pat. No. 5,403,641.
Crimp, however, is not limited to woven sailcloth and can occur with laid-up constructions also. Crimp in sailcloth made of laid-up yarn can be created in several different ways. First, lateral shrinkage of the films during many conventional lamination processes induces crimp into the yarns. For example, with narrow crosscut panel construction, where a majority of load-bearing yarns are crossing the panel widths, significant crimp of these yarns is induced during lamination of the sailcloth between high-pressure heated rolls. This is because the heated film shrinks laterally as it undergoes thermoforming, typically about 2.5% with this lamination method. The result is catastrophic with regard to the stretch performance for the composite fabric in highly loaded applications.
Second, uninterrupted load-bearing yarns within a sail follow curved trajectories. The yarns used are typically multifiber yarns. Twist is generally added so that the fibers work together and resist stretch along the curved trajectories. If no twist were added, only a few fibers would be submitted to the loads, that is the ones on the outside of the curve. This would substantially limit the ability of the sail to resist stretch. While the tiny yarn spirals created using the twisted multi-fiber yarns help increase load sharing amongst the fibers and therefore reduce stretch, there is still crimp induced as the spiraled yarns straighten under the loads. The twist in the yarns is therefore a necessary compromise for this design, preventing however this type of sailcloth from obtaining the maximum possible modulus from the yarns used.
The various approaches shown in Linville's patents are other attempts to reduce crimp problems. Layers of continuous parallel spaced-apart laid-up yarns are used to reinforce laminated sailcloth. However, because the continuous spaced-apart yarns are parallel to each other, only a small number of them are aligned with the loads. Panels cut out of these sailcloths therefore have poor shear resistance. In addition, no change of yarn density is achieved along the yarns direction. Therefore the proposed designs do not offer constant strain qualities. In addition, these approaches are designed to be used with panel-layout like the Cross-cut, Leech-cut and Tri-radial constructions, which result in their own sets of drawbacks.
The sailcloth shown in Meldner's patent may, in theory, reduce crimp problems. However, it is designed to be used in Tri-radial construction, which results in its own set of problems. Meldner laminates between two films continuous layers of unidirectional unitapes made from side-by-side pull-truded tows of filaments with diameters five times less than conventional yarns. The continuous unidirectional layers are crossing-over each other to increase filament-over-filament cross-over density, which is believed to minimize crimp problems and increase shear strength. Meldner is limited to the use of very small high performance yarns, which are expensive. The cost of those yarns affects greatly the economics of this approach and limits it to "Grand Prix" racing applications. In addition, this design of sailcloth is not intended to offer constant strain qualities; rather stretch and strength resistance are designed to be the same throughout the entire roll length of the sailcloth. Only a small number of the continuous unidirectional filaments end up aligned with the loads.
U.S. patent application Ser. No. 09/173,917 filed Oct. 16, 1998 and entitled Composite Products, Methods and Apparatus, describes a low stretch, flexible composite particularly useful for making high performance sails. The composite includes first and second polymer films with discontinuous, stretch resistant segments therebetween. The segments extend generally along the expected load lines for the sail. The segments have lengths which are substantially shorter than the corresponding lengths of the load lines within each sail section. The sail can be either two-dimensional or three dimensional. The two-dimensional sails can be made from one section or a number of flat sections seamed together. Three dimensional sails can be made using one or more molded sections of the composite sheet or several flat sections can be broad seamed together to create the three dimensional sail. The sail can be designed to exhibit generally constant strain qualities under a desired use condition and to permit low stretch performance to be optimized by minimizing the crimp, that is the geometrical stretch, of the yarns.